1. Field of the Invention
The present invention generally relates to a liquid crystal display and a computer, particularly but not limited to, an active matrix liquid crystal display suitable for the display of a dynamic image and a computer suitably used with the liquid crystal display. The present application is based on Japanese Patent Application No. 314274/2000, which is incorporated herein by reference.
2. Description of the Related Art
Recently, the display screen of a liquid crystal display (LCD) has been enlarged and the definition has been enhanced. A displayed image ranges from a static image as in a liquid crystal display used for a personal computer, a word processor and the like, to a dynamic image as in a liquid crystal display used for TV and the like. Since the compression technology for dynamic images has progressed and a dynamic image can now be also easily handled in a computer, a frequency at which a dynamic image is displayed also increases in a liquid crystal display used for a personal computer and the like. It is conceivable that as LCD is thin, compared with TV provided with a cathode ray tube (CRT), and can be installed without occupying a large place, the ratio of popularization of LCD TVs in the general home will increase in the future.
FIG. 1 is a block diagram showing the schematic structure of a liquid crystal display as a related art. In FIG. 1, a case that a computer 100 such as a personal computer and a liquid crystal display 110 are separately provided is shown as an example. As shown in FIG. 1, gradation data D100 and synchronism data D101 are output from the computer 100 to the liquid crystal display 110. For example, gradation data D100 means an RGB signal and synchronism data D101 is data including a vertical synchronizing signal, a horizontal synchronizing signal, a data enable signal (DE) and a clock.
The liquid crystal display 110 includes an LCD controller 112, a liquid crystal display panel 114, a signal line driving circuit 116, a scanning line driving circuit 118, a reference gradation voltage generator 120, a backlight 122 and an inverter for the backlight 124. The LCD controller 112 generates gradation data D110 and a signal side control signal D111 respectively output to the signal line driving circuit 116 based upon gradation data D100 and synchronism data D101 respectively output from the computer 100. The LCD controller 112 also generates a scanning side control signal D112 output to the scanning line driving circuit 118 and controls the displayed contents of an image displayed on the liquid crystal display panel 114.
Referring to FIG. 2, the structure of the liquid crystal display panel 114 will be described below. FIG. 2 shows an example of the structure of an active matrix LCD as a related art. Though the first and second glass substrates are not shown in FIG. 2, LCD is provided with the first and second glass substrates. On the first glass substrate, n scanning lines 131 (n: natural number) and m signal lines 132 (m: natural number) are arranged in a grid and a thin film transistor (TFT) 133 which is a nonlinear device (a switching device) is provided in the vicinity of each cross-point of the scanning line 131 and the signal line 132.
The gate electrode of TFT 133 is connected to the scanning line 131, the source electrode is connected to the signal line 132 and the drain electrode is connected to a pixel electrode 134. The second glass substrate is arranged in a position opposite to the first glass substrate and a common electrode 135 is formed on one surface of the glass substrate by a transparent electrode such as an indium-thin-oxide (ITO) film. Each common electrode 135 is connected to a common electrode driving circuit 136 and the electric potential is set by the common electrode driving circuit 136. Liquid crystal is filled between the common electrode 135 and the pixel electrode 134 formed on the first glass substrate.
The scanning line 131 and the signal line 132 are respectively connected to the scanning line driving circuit 118 and the signal line driving circuit 116. The scanning line driving circuit 118 executes scanning by sequentially applying high potential to n scanning lines 131 and turns on the TFT 133 connected to each scanning line 131. Gradation voltage is written to the pixel electrode 134 via turned-on TFT 133 when the signal line driving circuit 116 outputs gradation voltage. The signal line driving circuit 116 outputs gradation voltage according to image data to one of the m signal lines 132 while the scanning line driving circuit 118 turns on the scanning line 131. The amount of transmitted light is controlled based upon potential difference between the common electrode 135 set to a fixed potential and the gradation voltage written to the pixel electrode 134.
As shown in FIG. 1, the liquid crystal display 110 is provided with the backlight 122 and the inverter for the backlight 124 for supplying power to the backlight 122. As the backlight 122 emits light at fixed luminance in a state in which the liquid crystal display 110 is operated, the amount of transmitted light emitted from the backlight 122 is controlled and display is made according to the above-mentioned operational principle. The reference gradation voltage generator 120 shown in FIG. 1 supplies reference gradation voltage to the signal line driving circuit 116.
FIG. 3 shows the waveforms of signals output from the scanning line driving circuit 118 and the signal line driving circuit 116, which are respectively provided to the scanning line 131 and the signal line 132 in the conventional type liquid crystal display. In FIG. 3, an x-axis shows time and VG1 to VGn respectively show the waveform of a scanning signal applied to each scanning line 131. As shown in FIG. 3, high potential is applied to only one scanning line 131 at a time and the scanning signals VG1 to VGn are signals sequentially output to n scanning lines 131. VD shows the waveform of a signal output to one signal line 132 and Vcom shows the waveform of a signal applied to the common electrode 135. In an example shown in FIG. 3, the signal strength of the signal VD varies according to each image data, and the signal Vcom has a fixed value and is a signal which does not vary with time.
The liquid crystal display of the related art and its driving method are described above. However, in the liquid crystal display of the related art, voltage applied to each pixel electrode 134 is held until the scanning line is selected next, thereby fixing transmitted light for one frame period. In the meantime, CRT sequentially scans using an electronic beam. In case a dynamic image is displayed on this LCD, a problem occurs in that the image quality deteriorates due to causes such as a residual image phenomenon. The cause of this deterioration is thought to be because the speed of a response of liquid crystal material is slow. As a result, when gradation varies, the variation of gradation cannot be completed in one field period and an accumulative response is performed in a few field periods. One approach to preventing this deterioration has involved the research of various liquid crystal materials that enable a high speed response.
However, it has been reported that the above-mentioned problems such as the residual image phenomenon are not caused by only the speed of the response of liquid crystal, but are also caused by the methods used to change the image displayed on LCD. Such reports have been made by NHK Broadcast Technical Research Institute and others (for example, refer to pp. 207 and 208 of SC-8-1 at 99' General Meeting of The Institute of Electronics, Information and Communication Engineers). To address the problems caused by the conventional methods used to change the image displayed on LCD, a method of driving CRT and a method of driving LCD will be described and compared below.
FIGS. 4A and 4B show the result of comparison between CRT and LCD in the time response of display light at a certain pixel. FIG. 4A shows the time response of CRT and FIG. 4B shows the time response of LCD. As shown in FIG. 4A, CRT is a so-called “impulse display” that emits light only for a few milliseconds since an electronic beam reaches a fluophor on the surface of the tube, while LCD shown in FIG. 4B is a so-called “hold-type display” that holds display light for one field period since the writing of data to a pixel is finished until the next writing is started.
When a dynamic image is displayed on CRT and LCD respectively having such characteristics, display as shown in FIGS. 5A and 5B are made. FIGS. 5A and 5B show examples of when a dynamic image is respectively displayed on CRT and LCD, FIG. 5A showing the example of CRT, and FIG. 5B showing the example of LCD. In FIGS. 5A and 5B, a circular display object moves in a direction shown by x. As shown in FIG. 5A, a display object is instantaneously displayed in a position corresponding to time on CRT, while an image before one field remains until immediately before new writing on LCD.
In case a person looks at a dynamic image displayed as shown in FIGS. 5A and 5B, the dynamic image is viewed as shown in FIGS. 6A and 6B. FIGS. 6A and 6B are explanatory drawings for explaining an image viewed by a person when a dynamic image is displayed on CRT and LCD, FIG. 6A showing a case of CRT and FIG. 6B showing a case of LCD. As shown in FIG. 6A, when a dynamic image is displayed on CRT, it is never viewed that an image displayed at a certain time is overlapped with an image from before that time. However, when a dynamic image is displayed on LCD, an image currently displayed is viewed in a state in which it, and an image displayed immediately before it, are overlapped due to the visual time integral effect and other effects, causing movement to become dim.
Some methods have been proposed for reducing the hold time of the displayed image by inputting voltage corresponding to a black image, prior to inputting voltage according to image data into each pixel electrode 134 of the liquid crystal display panel 114. Such proposed methods prevent movement from being dim, thereby solving the above-mentioned problems caused when a dynamic image is displayed on LCD. FIGS. 7A to 7D are explanatory drawings for explaining a method of preventing movement from being dim by inserting a black image between each image data. This method basically prevents movement from being dim by applying predetermined voltage for black display to liquid crystal for a horizontal blanking period as shown in FIG. 7A. That is, after an image in one field is displayed, black is displayed on the whole screen and an image in the next field is displayed. However, according to this method, as display time is different for each respective scanning line of the liquid crystal display panel 114. This difference in display time causes a problem in that luminance difference depends upon the location on the liquid crystal display panel 114 as shown in an example in FIG. 7C.
A method of preventing luminance difference from being caused is proposed in Japanese published unexamined patent applications No. Hei. 9-127917, No. Hei. 10-62811 and Japanese published unexamined patent applications No. Hei. 11-30789. FIG. 8 shows the structure of a liquid crystal display to solve the problem caused by the method shown in FIG. 7A. This structure is proposed in the above-mentioned patent application No. Hei. 9-127917. The same reference number is allocated to the same member as that in the liquid crystal display shown in FIG. 2.
In FIG. 8, in addition to the circuit structure shown in FIG. 2, a circuit for writing black is newly provided, including: a black signal feeder 140, a black signal supply line 141, a scanning line for supplying a black signal 142, TFT for supplying a black signal 143 and a scanning line driving circuit 144 for driving the scanning line 142 for supplying a black signal. The gate electrode of the TFT for supplying a black signal 143 is connected to the scanning line for supplying a black signal 142, the source electrode of the TFT for supplying a black signal 143 is connected to the black signal supply line 141 and the drain electrode is respectively connected to the drain electrode of TFT 133 and the pixel electrode 134.
In the liquid crystal display having the above-mentioned configuration, voltage corresponding to black is applied to the pixel electrode 134 in one field, and afterward, voltage according to image data is applied to the pixel electrode 134. Image data is reset by independently driving each scanning line as described above, and illustrated in the example shown in FIG. 7B. That is, difference in luminance is prevented by resetting each scanning line independently, inserting black after each image is displayed, instead of resetting all of the scanning lines simultaneously as shown in FIG. 7A. By resetting the scanning lines as shown in FIG. 7B, the screen luminance differences can be prevented, as shown by the panel display in FIG. 7D.
However, the deterioration of image quality such as a flicker occurs due to a black screen inserted also in the display of a static image, both in the method shown in FIGS. 7A and 7C and in the device shown in FIG. 8, even though a hold-type display is suited for the display of the static image. Also, because the brightness of a display screen is reduced when a black screen is inserted, the luminance of the backlight is required to be set to a high value so as to acquire the brightness of the same extent as the brightness acquired in case no black screen is inserted. This increase in the luminescence of the back light increases power consumption, which is also a problem.
A liquid crystal display in the present invention may display a dynamic image without dim movement or without the deterioration of luminance. A liquid crystal display in the present invention also may display a static image without needless power consumption or without the deterioration of image quality such as a flicker. Further, a computer suitably used with the liquid crystal display is provided by the illustrative embodiment of the present invention.